RFR: 8221396: Clean up serviceability/sa/TestUniverse.java

[Per Liden]

Clean up serviceability/sa/TestUniverse.java to remove the need for the 
withZ/withoutZ option we currently pass in. This also changes the test 
to only run with the selected GC instead of testing all GCs every time, 
which should save us some cycles in testing.

Bug: https://bugs.openjdk.java.net/browse/JDK-8221396
Webrev: http://cr.openjdk.java.net/~pliden/8221396/webrev.0

/Per